Pellets containing drugs of different properties were prepared in a Rotoprocessor in order to study changes in the formulation process and resulting pellet characteristics. Diltiazem hydrochloride, diclofenac sodium, and theophylline were chosen as model drugs. Pellet size distribution, sphericity, density, hardness, friability, and repose angle were determined using standard methods. The amount of water as a wetting agent necessary for successful pellet formulation was observed for each sample and changed depending on drug solubility, concentration, and particle size. The pelletization of freely soluble diltiazem hydrochloride required 24.8-23.1% of the wetting agent and its amount decreased as the drug concentration increased. The demand for water in the formulation of theophylline pellets was 31.0-34.4% and it increased with increasing drug concentration. The pellet samples containing both drugs were easy to prepare. However, the cohesion of micronized diclofenac sodium particles negatively influenced both the pellet size distribution and the formulation process itself. When the drug concentration exceeded 40%, it was not possible to produce pellets of an appropriate size and the process was not reproducible.